Thanks to multi-channeled broadcast and developing video services through communications, the number of programs which are available for general users has been increasing. However, the users' spare-time for watching the programs is limited within a day. Accordingly, the users cannot always watch every program that they are interested in.
Recently, a portable viewing/listening system which enables the users to watch programs in their spare time (during lunch or in transit) has been proposed. A conventional portable viewing/listening system is constituted by a main device which is installed in a house and an easy-to-carry portable device. The main device is at least capable of receiving program data that is provided through broadcasting, internally-storing the received program data, and transferring the stored program data to the portable device.
The conventional portable viewing/listening system, however, takes a long time to transfer the program data from the main device to the portable device. Accordingly, if the users run out of time and thus stop data transfer halfway through, the program data is not entirely transferred, and thus the users cannot enjoy the entire program.
FIG. 18 is a diagram illustrating a data transfer mode which is adopted in the conventional portable viewing/listening system. Hereinafter, by referring to FIG. 18, problems that are associated with the conventional system are described in more detail.
FIG. 18(a) shows a time transition of data constituting a program to be broadcast. In FIG. 18(a), a lateral axis indicates a lapse of time during broadcast, and a vertical axis indicates a bit rate during broadcast, respectively. To be more specific, FIG. 18(a) shows that Program 1 is broadcast between Time t0 and Time t1, Program 2 is broadcast between Time t1 and Time t2, and Program 3 is broadcast between Time t2 and Time t3, respectively. The Program 1, Program 2, and Program 3 are respectively provided with audio, closed caption, and video as program components (mono-media constituting a program). For the sake of clarity, it is herein assumed that a length of broadcast time for the Program 2 (t2−t1) is equal to the length of broadcast time for the Program 3 (t3−t2), and a length of broadcast time for the Program 1 (t1−t0) is exactly twice the length of broadcast time for the Program 2 (or the Program 3). It is also assumed that a bit rate for the respective program components remains invariant no matter which Program, and thus a ratio among a bit rate of closed caption bc, bit rate of audio ba, and bit rate of video by is always 1:2:4.
FIG. 18(b) shows a time transition which takes place when the program data is transferred from the main device to the portable device in the conventional portable viewing/listening system. In FIG. 18(b), a lateral axis indicates a lapse of time in a data transfer operation, and a vertical axis indicates a bit rate during transfer, respectively. As shown in FIG. 18(b), transfer of the program components of the Program 1, Program 2, and Program 3 is completed between Time t4 and Time t9. In FIG. 18(b), unlike the bit rate during broadcast shown in FIG. 18(a), the bit rates for the program components during transfer are each compressed or extended in the direction of a time axis according to characteristics of data for the respective program components. However, the program components herein are basically transferred in the same order as those during broadcast. This means that every program component of the Program 1 is transferred between Time t4 and Time t7, every program component of the Program 2 is transferred between Time t7 and Time t8, and every program component of the Program 3 is transferred between Time t8 and Time t9, respectively. A ratio among (t7−t4), (t8−t7) and (t9−t8), all of which indicate a length of time which is taken to transfer each Program, is 2:1:1. This is equal to the ratio among (t1−t0), (t2−t1) and (t3−t2), all of which indicate a length of broadcast time for each Program. Assuming that a total bit rate for closed caption, audio and video during transfer is a -time as much as a total bit rate during broadcast, a ratio between a length of broadcast time of a certain program and a length of time which is taken to transfer the program is always α:1.
FIG. 18(c) shows a case where a transfer of program data is discontinued halfway at Time to in the conventional portable viewing/listening system. In this example, the Time ta is assumed to satisfy t7<tα<t8. As transfer of the program data is discontinued in the middle of the Program 2 between Time t7 and Time t8, the entire data of the Program 1 and a partial amount of the data of the Program 2 from its beginning to Time tα is transferred to the portable device. However, as to the remaining portion of the data of the Program 2 and the entire data of the Program 3, none of the program components, i.e., closed caption, audio and video, included therein is transferred to the conventional portable viewing/listening system. As to the not-transferred part of data, the portable device does not inform the users what the non-transferred data is. Therefore, the users cannot even know whether or not the non-transferred data is interesting enough to watch.